The invention relates to a switching device which is arranged to switch a load to direct voltage. The switching device of the invention is used in applications where a high capacitive load is switched to direct voltage, in which case the charging current of the capacitive load needs to be limited.
A system which has a high capacitive load and is connected to a direct current circuit has to be protected during voltage switch-on. If the capacitors of the system are not charged, sudden switch-on of direct voltage generates a surge current, which may damage system components switched behind the switch. Usually the system is protected by using a three-position and three-pole switching device which supplies the charging current of the capacitive load through a charging resistor to the load during charging. The charging resistor limits the charging current and prevents problems that sudden charging may cause in the system. After charging the charging resistor is bypassed by switching the load directly to direct voltage. An example of a device with a high capacitive load is a frequency converter/converters which is/are switched to a direct voltage supply circuit.
One prior art solution is shown in FIG. 7. This solution employs a manual three-position switching device for switching a high capacitive load to direct voltage. The switching device comprises a three-position control member which is controlled by turning the handle attached to the control shaft of the control device. During charging the current is supplied to charging resistors via an array of auxiliary contacts. The positions of both the switching device and the control member are Open [0], Charging [C] and Closed [1]. Turning of the control member and the switch from the switching position into the closed position is prevented by a limiter, such as a magnetic switch. The control member and the switch cannot be turned into the closed position until the limiter allows this. The operation of the limiter is controlled by a separate control signal, which is dependent on the charging state of the capacitive load.
A problem related to the prior art switching device is that the control device of the switching device has three positions and an array of auxiliary contacts. This means that controlling of the switching device includes a phase which is unnecessary for switching, i.e. charging phase, which requires that the person performing switching manually keeps the switching device in the charging [C] position until the limiter allows turning of the switch into the closed [1] position. This makes switching more difficult and involves the risk of misuse of the switching device if the person performing switching is unfamiliar with the special functions of the switching device. Secondly, the array of auxiliary contacts comprises a large number of moving mechanical parts which are easily damaged.
The object of the present invention is to eliminate disadvantages of the prior art by providing an improved switching device where the operation of the switching member corresponds to that of a three-position switching device but whose structure provides a two-position control device with only the positions open [0] and closed [1]. In that case the use of the switching device in switching is considerably more natural to switch of this kind as well as more reliable and safer.
This is achieved by a switching device which is characterized by what is disclosed in the claims.
The switching device is arranged to switch a load to a direct voltage circuit. It comprises a switching member consisting of at least three contacts, a control member which controls the contacts of the switching member and has the positions open [0] and closed [1], one or more spring members which control the contacts and are tensioned by the control member, and the switching device is characterized in that the switching member of the switching device comprises a limiter arrangement which is arranged to limit the closing of at least one contract when the control member is turned from the open [0] position into the closed [1] position, the limiter arrangement comprising a limiter, a toggle joint connected thereto and a solenoid arranged in the toggle joint, which is arranged to be controlled by the solenoid, as a result of which the toggle joint turns the limiter so that it releases the contact of the switching member whose closing has been prevented, allowing the contact to close.
The preferred embodiments of the invention are disclosed in the dependent claims.
For charging a capacitive load, the switching device is provided with a limiter arrangement, i.e. a mechanism which limits the movement of the third contact of the switching device when the control device is turned from the open [0] position into the closed [1] position and prevents the third contact of the switching member from closing when the control device is turned. The mechanism leaves the contact open to wait for release in an excited state generated by spring force. The contact is released after a certain charging state of the capacitive load has been reached.
The mechanism consists of a limiter, which is preferably a lever arm, and of a toggle joint attached thereto. The first end of the lever arm is connected to the toggle joint and the second end is connected to the control member of the third contact in the switching device. The toggle joint comprises a fold where an electrically-controlled solenoid is arranged. The solenoid generates a pulse in the fold of the toggle joint so that the toggle joint can be folded and the spring tension released, which allows the third contact to close.
In an embodiment the joint at the first end of the limiter, where the limiter is connected to the shaft of the control device, comprises a clearance. The clearance allows the shaft to rotate by a certain angle without moving the limiter. This way the spring member of the switching device is tensioned for release but turning of the shaft does not influence the position of the limiter. The clearance is preferably implemented as follows: the first end of the limiter, where the limiter is connected to the shaft of the control device, is provided with an opening through which the shaft is guided. The opening is shaped to allow rotation of the shaft by a certain angle without moving the limiter, as a result of which the spring member of the switching device is tensioned for release without the turning of the shaft affecting the position of the limiter.
In another embodiment all contacts of the switching device are controlled by a common shaft. In that case the necessary clearance can be implemented by controlling the first and the second contact with a common shaft. At the third contact the shaft comprises a connecting sleeve for implementing the clearance.
A sensor, such as a voltage sensor, is arranged to sense the charging state of the capacitive load and to control the solenoid.
The switching device of the invention thus has three operating positions, i.e. open [0], charging [C] and closed [1], whereas the control member of the switching device preferably has only two positions, i.e. open [0] and closed [1]. This makes the switching device simple and safe to use since the control member of the switching device needs to be turned only between the open [0] position and the closed [1] position thanks to the fact that the mechanism of the invention provides the switching device with automatic operation for controlling the operation of the switching device between the positions charging [C] and closed [1].
A further advantage of the solution is that no auxiliary contacts are needed in the switching device implemented according to the invention, and consequently the structure of the device is simpler and more reliable.